Unheralded Austrian Bernd Wiesberger carded his second successive 68 for a share of sixth with five-time major champion Mickelson, who eagled the last for a 67.

Westwood was going along nicely until a double-bogey at the 13th saw him drop down the leaderboard

United States Ryder Cup vice-captain Steve Stricker, South Africa's former Open champion Louis Oosthuizen and Sweden's Henrik Stenson are tied for 11th on five under, alongside Westwood.

"I didn't get off to the best of starts but I righted the ship well, caught fire a little bit round the turn and am really happy with four under par," said McIlroy, who has carded 12 successive sub-par scores.

"I'm feeling good about my game, I'm confident and hitting the ball well for the most part. I'm in control of my game and my emotions and I will need to do that over the weekend as well."

"Rory McIlroy is realising his immense potential having immersed himself in golf since ending his engagement to Caroline Wozniacki in May. His level of confidence and consistency is redolent of Tiger Woods at his best. Since his last 'Freaky Friday', when he shot 78 in the second round of the Scottish Open, this has been his scoring sequence: 66, 67, 66, 66, 68, 71, 69, 64, 66, 66, 66 and 67. This is why he is perfectly placed to claim his second major of an increasingly remarkable season."

Only 20 minutes after the first group teed off, play was suspended for almost an hour because of torrential rain in Louisville, Kentucky.

But when play resumed, 2012 champion McIlroy, who is bidding to become the first man to win back-to-back majors since Ireland's Padraig Harrington in 2008, made light of the odd muddy ball.

He eagled the 18th, his ninth hole, to go out in 33 and had chances on his back nine to make his score even better than it was.

He missed a makeable eagle putt at the seventh after a magnificent five-wood approach from 240 yards, before leaving a birdie putt just short at the par-three eighth.

But he curled in a 16-foot birdie putt at the ninth to put daylight between himself and the field.

Mickelson, champion in 2005, picked up an eagle at the par-five 18th to move to within three of the lead

Some players who teed off early were critical of the decision to start the day's play in such inclement weather, then leave them out on the course in the rain and not allow preferred lies.

"I wasn't quite sure if we should have teed off to be honest with you," said Palmer, the world number 63.

"You could barely see the fairway. And we were questioning why we had to hit into a puddle because the first green was already under water."

Australia's world number two Adam Scott is two under after a second-round 69, while Bristol's Chris Wood, who was one shot off the lead overnight, is three under after a 73.

Jamie Donaldson, the only Welshman in the field, is also three under after a 70, while England's Ian Poulter, who is hunting a spot at the Ryder Cup, is one under after a 73.

Day has seven top-10 finishes in only 14 appearances in major tournaments

Masters champion Bubba Watson, England's former world number one Luke Donald and Spain's Sergio Garcia, runner-up to McIlroy in his last two events, carded 72s for level par overall.

Also level par is England's Justin Rose (72), while Northern Ireland's Graeme McDowell, also a former US Open champion, birdied the 17th for a 70 to make the cut, which fell at +1, by one shot.

Scottish veteran Colin Montgomerie, who qualified for the tournament by winning this year's Senior PGA Championship, also made it to the weekend.

But Harrington, who narrowly missed an eagle putt at the last, and Germany's US Open champion Martin Kaymer both missed the cut by one shot.

Furyk, winner of the US Open in 2003, birdied the final two holes to get to within one shot of the lead